Radio Frequency Identification (“RFID”) is a generic term for technologies that use radio waves to automatically identify individual items. Objects can be identified using RFID by storing a serial number that identifies the object on a chip that is attached to an antenna. The chip and the antenna together are called an RFID tag. An RFID reader sends out electromagnetic waves that are received by the antenna on the RFID tag. Passive RFID tags draw power from this electromagnetic field to power the chip. Active tags use their own batteries to power the chip. The tag responds to the reader by transmitting a bit stream to the reader that contains information about the tag (serial number, etc.). The current state of RFID technology is described in [1] K. Finkenzeller, “RFID Handbook” (John Wiley & Sons, 2003). Background information regarding the present disclosure may be found in [2] U.S. patent application Ser. No. 11/066,048, “Method and Apparatus for Improving the Efficiency and Accuracy of RFID Systems,” filed Feb. 25, 2005, which is hereby incorporated by reference in its entirety.
RFID systems operate at many different frequencies. The most common frequencies are low frequencies around 135 KHz, high frequencies around 13.56 MHz, ultra-high frequencies (UHF) around 900 MHz, and microwave frequencies around 2.45 GHz and 5.8 GHz. Global protocol standards ensure compatibility between RFID systems from different manufacturers. For example, EPCglobal, Inc. has developed a protocol for UHF systems described in [3] “EPC™ Radio-Frequency Identity Protocols Class-1 Generation-2 UHF RFID Protocol for Communications at 860 MHz-960 MHz” (EPCglobal, Inc., 31 Jan. 2005). Tags that adhere to this protocol transmit information only after receiving specific instructions and can be instructed to stay quiet with a “sleep” command, which is in effect until a “wake” command is issued. Thus, a reader interrogates only the subset of the tag population that has not received the sleep command. (The “sleep” and “wake” commands as described here can be obtained by combining commands specified in [3].)
Current RFID systems do not have well-defined interrogation zones, and tags in large regions are set off unintentionally. Reference [2] discloses RFID readers that transmit data signals that cause the tags to respond and scramble signals that do not cause the tags to respond. The data and scramble signals are transmitted with different beams that are adjusted such that the scramble signals overshadow the data signals in all but selected regions. Hence, a tag will respond only if it is located in one of the selected regions, called the interrogation zones.